Work roll for hot rolling

ABSTRACT

A work roll for hot rolling comprising a body portion composed of a steel having a ferrite structure and having a yield strength at least 40 kg/mm2, improvement wherein substantially the whole surface of said body portion in the axial direction is provided with a weld metal having a high smoothed surface and having a thickness of 7 to 50 mm, said weld metal consisting essentially of 0.2 to 0.8 wt% of C, 4 to 10 wt% of Cr, 1 to 4 wt% of Mo, 0.5 to 5 wt% of V, 0.1 to 1.5 wt% of Si, 0.1 to 3 wt% of Mn, up to 0.5 wt% of Ni, up to 0.015 wt% of P, up to 0.015 wt% of S, at least one of 0.01 to 0.5 wt% of Ti and 0.05 to 1 wt% of Al, and the balance of substantially Fe, said components other than Ti and Al showing a hot crack sensitivity value of 2.7 or less when calculated according to the formula,

Unite States Patent Nemoto et a1.

['11] 3,855,015 [4 1 Dec. 17,1974

1 WORK ROLL FOR HOT ROLLING [75] Inventors: Tadashi Nemoto, Takahagi; Kiyoshi Watanabe, Hitachi; Tadaaki Kobayashi, Hitachi; J00 lshihara, Hitachi; Kenji Ono, Tokyo; Kazuhiko Sonomoto, Kitakyushu, all of Japan [731 Assignees: Hitachi, Ltd.; Hitachi Metals, Ltd.,

. Tokyo, Japan 22 Filed: May 4, 1972 21 Appl. No; 250,186

Related US. Application Data [63] Continuation-in-part of Ser. No. 86,406, Nov. 3,

1970, abandoned.

[] Foreign Application Priority Data Nov. 4, 1969 Japan 44-87559 [52] US. Cl. 148/34, 29/1961 [51] Int. Cl C04b /00, B32b 15/00 [58] Field of Search 75/124, 126 E, 126 D; 148/3 4,36; 29/1961 [56] References Cited UNITED STATES PATENTS 2,069,260 2/1937 Merten 75/126 E 2,572,191 10/1951 Payson 75/126 2,590,835 4/1952 Kirkby 75/126 E 2,848,323 8/1958 Harris 75/126 D Fletcher Primary Examiner-Hyland Bizot Attorney, Agent, or FirmCraig & Antonelli 5 7 ABSTRACT A work roll for hot rolling comprising a body portion composed of a steel having a ferrite structure and having a yield strength at least kg/mm improvement wherein substantially the whole surface of said body portion in the axial direction is provided with a weld metal having a high smoothed surface and having a thickness of 7 to mm, said weld metal consisting essentially of 0.2 to 0.8 wt% of C, 4 to 10 wt% of Cr, 1 to 4 wt% of M0, 0.5 to 5 wt% of V, 0.1 to 1.5 wt% of Si, 0.1 to 3 wt% of Mn, up to 0.5 wt% of Ni, up to 0.015 wt% of P, up to 0.015 wt% of S, at least one of 0.01 to 0.5 wt% of Ti and 0.05 to 1 wt% of Al, and the balance v of substantially Fe, said components other than Ti and Al showing a hot crack sensitivity value of 2.7 or less when calculated according to the formula,

and said weld metal having such a structure that carbide particles have been distributed in a martensite matrix.

9 Claims, 6 Drawing Figures wnwan 1010 wr/ /z/ lfllllllllllllll/llll/I/l/ll/I/l/ll/l/ PAIENTEDHEWTTW K 3.555.015

sum 1 of 3 PRESENCE- PRESENCE OF CRACKS H. 6.3. VALUE PRESENCE PRESENCE OF CRACKS H.C.S.

CRACKS 96} TENTED 3,855,015

sum-29513;

' L A CRACKS o WELD MEmL WHEN //?0/V P0n0ERs ARE ADDED I I I AMOUNTS 0F ALLOY/N6 ELEMENTS {W796} PATENTED BEE 71974 SHLET 3 BF 3 5 mafia W TJ 0 -0 6 0 -0 5 0 -0 4 6 G W a m m 00 5 www TEMPE/PING TEMPERATUPE PC} WORK ROLL FOR HOT ROLLING CROSS-REFERENCE TO THE RELATED APPLICATION:

This is a continuation-in-part of our copending application, Ser. No. 86,406 filed on Nov. 3, 1970, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a work roll for hot rolling of such a construction that a weld metal which is excellent in rolling characteristics and low in weld crack sensitivity has been formed over the whole surface of an arbor material.

2. Description of the Prior Art l-lot rolling is an operation carried out by heating a metal or alloy to above the recrystallization temperature thereof to increase the metal or alloy in plasticity and passing the heated metal or alloy between rolls rotating at normal temperature or at an elevated temperature to shape the same into plates, rods or structural shapes, thereby destroying the cast structure of the metal or alloy to impart uniform and excellent properties thereto.

The rolls of a rolling machine are composed of a work roll which directly rolls and deforms a metal or alloy, and a back-up roll which reinforces the said work roll. In the rolling of a metal or alloy, the work roll is brought to the most severe state. It is well known that the work roller is easily worn out since it contacts with a material to be rolled which has been heated to'a high temperature, tends to form cracks on the surface due to thermal fatigue failure since it is repeatedly subjected to thermal stress, and tends to be roughened on the surface since cracked portions formed on the roll surface are fallen off or fused. Accordingly, the material to be used for manufacture of the work roll should be high in resistance to the above-mentioned phenomena. In addition, the work roll is required to be high in strength and sufficient in toughness, since the ratio of reduction in hot rolling which is observed at the time when a material to be rolled in inserted between rolls is high and the work roll is liable to be bent. The wear resistance, hot cracking resistance and surface roughening resistance are indispensable matters for the work roll, and are generally called rolling characteristics.

In order to impart the rolling characteristics of a high level to the work roll, the material to be used for manufacture of the work roll should be increased in hardness as far as possible. On the other hand, however, the work roll is required to be high in tenacity as well. Since hardness and tenacity are in a relation contrary to each other, there is naturally a certain limit in imparting excellent rolling characteristics to one roll. For

the above reasons, the work roll of the prior art has quently become causes for surface-roughening due to the falling-off of portions where the pin-holes have been formed. Accordingly, weld metal materials are also limited to specific materials.

As the result of extensive studies, the present inventors have found that as the weld metal to beformed on the surface .of a body material for a work roll for hot rolling, a weld metal having a martensite structure which contains about O.2 to 0.8 by weight of C is most preferable among those regulated in ASTM 399-56T. This weld metal, however, frequently formed many hot cracks when formed on the surface of a roll body by an automatic welding operation, which is ordinarily adopted for hardfacing of the roll body. Accordingly, the inventors have investigated the cause therefor to find that during the course of solidification of the weld metal from its molten state, such elements as C, P and S, which are marked in segregation tendency, are further concentrated to the grain boundary when the weld metal is subjected to automatic welding operation, which requires a greater heat input and a longer solidification time, and the said elements separate from 1 each other due to external and internal restriction forces at the concentrated portion to form cracks, and that such elements as Cr and Mn have actions to inhibit the weld metal from cracking even when the. amount of C is left unchanged. The above findings coincide with the effects on hot crack sensitivity of various components of the high tensile alloy steel which was proposed by F. J. Wilkinson et al. [see F. J. Wilkinson, C. L. M. Cottrel: British Welding Journal, 1958 (12), pages 557-562]. That is, they proposed the equation H.C.S. C(P s si/25 Ni/)/3Mn Cr lVIo (wherein H.C.S. is an abbreviation for hot crack sensitivity), and clarified that according to tungsten inert gas shielded arc welding, as weld metal having an H.C.S. value of 4 or less is high in resistance to hot cracking. The effects of various components used in the above-mentioned weld metal coincided with the results obtained by the present inventors. The present inventors applied to the above-mentioned equation the results of experiments on a weld metal containing 0.20

to 0.60 by weight of-C, 1 to 12 by weight of Cr,-

and, in addition thereto, Si, Mn, Ni, Mo, V, etc. which had been formed by submerged arc welding, to find that the relation between the presence of cracks and the I-I.C.S. value is as shown in FIG. 1, wherein the black points indicate an Ni-free weld metal. From FIG.

1, it appears that the absence of Ni in the weld metal has broadened the transition zone (I-I.C.S. value 1.2 to 2.1) with respect to the crack formation. Accordingly, in case the weld hot crack sensitivity observed in an automatic welding operation isappliedto the equation proposed by Wilkinson et al., there is a question in the Ni coefficient adopted in said equation. According to the experience of the present inventors, it is considered that said Ni coefficient is too small. In view of this, the inventors investigated the range of distribution of I-I.C.S values by varying the Ni coefficient to find that the Ni coefficient is adequately U30 and, in this case, the transition zone for crack formation can be limited to such a narrow l-I.C.S. value range as from 1.6 to 1.8, as shown in FIG. 2.

However, the limitation that the modified H.C.S. value is less than 1.6 is extremely severe because in the case of a work roll or the like, which is required to be particularly high in surface roughening resistance, it is desired that the content of C in the weld metal used is increased within a possible range. That is, even when judged sensibly, it may be said that in case the content of C in a weld metal is 0.5 by weight for example, the weld metal cannot be inhibited from hot cracking unless the value of 3Mn Cr Mo V is increased to about 20 by weight, and thus various problems occur in practice.

As the result of studies on such various restrictions as mentioned above, the present inventors have found that the limit of the modified I-I.C.S. value can be broadened to 2.7 by alloying a proper amount of Ti and/or A1 with the weld metal material.

SUMMARY OF THE INVENTION 1. Objects of the Invention A principal object of the present invention is to provide a work roll for hot rolling of such a construction that a weld metal, which is extremely low in weld crack sensitivity and excellent in rolling characteristics, has been formed on the surface of an body material.

Another object of the invention is to provide a work roll for hot rolling of such aconstruction that a weld metal, which is greatly inhibited in weld hot cracking at the time of automatic welding operation, has been formed on the surface of an body material.

2. Statement of the Invention The work roll for hot rolling which is provided ac cording to the present invention comprise a body portion composed of a steel having a ferrite structure and having a yield strength at least 40 kg/mm improvement wherein substantially the whole surfaceof said body portion in the axial direction is provided with a weld metal having a high smoothed surface and having a thickness of 7 to 50 mm, said weld metal consisting vTi and Al showing a hot crack sensitivity value of 2.7

or less when calculated according to the formula,

C(P S Si/25 Ni/30)/3Mn Cr -l- Mo V X 1'0 and said weld metal having such a structure that carbide particles have been distributed in a martensite matrix. v

The work roll according to the present invention is such that on the surface of a body has been formed a weld metal which is excellent in rolling characteristics and which does not cause weld hot cracking, so that the body may have been made of any material so far as it can withstand the bending force applied at the time of rolling. According to many experiments, such body material is preferably a steel having a ferrite structure high in ductility and having a yield strength of at least 40 kglmm 1f the body which contacts with a material to be rolled has no excellent and uniform rolling characteristics all over the surface, the two sides of the material to be rolled differ from each other in degree of deformation or the roll is locally damaged. Accordingly, the weld metal should be formed to-a uniform thickness over the whole surface of the body.

In order to form a weld metal excellent in rolling characteristics, it is necessary to regulate the thickness of the weld metal. If the thickness of the weld metal formed is less than 7 mm, the weld metal cannot display high wear resistance, while if the thickness is more than 50 mm, the number of steps required for formation of the weld metal becomes larger to bring about economical disadvantages.

The weld metal formed according to the present invention displays excellent rolling characteristics, i.e., high hot cracking resistance, wear resistance and surface roughening resistance. Further, the weld metal has been so controlled in composition that the effect of hot cracking prevention due to the presence of Tiand/or Al can sufficiently be displayed. Even when subjected to automatic welding operation, therefore, the weld metal does not bring about any such weld hot cracking as to be argued in practice, so far as the modified H.C.S. value thereof is up to 2.7. I

Among the constituents of the weld metal, Cr dissolves in a major proportion into the matrix at the time of quenching to improve the quenching property thereof. Further, Cr is effective for prevention of hot cracking as well. From the standpoint of the quenching property improvement, Cr is required to be used in an amount of at least 4 by weight. If the amount exceeds 10 by weight, however, an eutectic of Cr C is developed in the form of network, with the result that hot cracking tends to take place. Accordingly, the amount of Cr should be made less than 10 by weight. In order to obtain a weld metal which scarcely brings about hot cracking and which is excellent in quenching property, the amount of Cr to be used is preferably 6 to 9 by weight.

Mn is also effective for prevention of hot cracking, like Cr, but if the content of Mn exceeds 3 by weight, the retained austenite formed in the tissue of the weld metal is stabilized to make it difficult to apply thereto such heat treatments as quench-hardening and temperhardening. The amount of 0.1 by weight is the minimum amount of Mn to display its effect. Preferable range of Mn content is from 1 to 2 by weight. So far as the content of Mn is within the above-mentioned range, the retained austenite formed in a welded state can easily be converted into martensit'e by subsequent heat treatment, and the effect of hot cracking prevention can effectively be displayed.

Si improves the oxidation resistance of the weld metal, and this effect is displayed when Si is used in an amount of at least 0.1 by weight. However, Si increases the modified H.C.S. value, and hence is preferably used in a smaller amount. The amount of 1.5 by weight is the critical amount of Si to inhibit the modified l-l.C.S. value to less than 2.7.

V forms, at the time of welding, a cluster'together with C (which means such a state that the particles of V have clustered around the C) in the weld metal in a molten state to prevent hot cracking and, after solidification, bonds to C in an amount of about one-fifth to one-fourth the weight of V to form a vanadium carbide, thereby contributing to the improvement in wear resistance of the roll. A part of the vanadium carbide dissolves in the matrix at the time of quenching, and deposits and hardens at the time .of tempering to increase the high temperature hardness of the roll. if the amount of V is less than 0.5 by weight, the amount of V which dissolves in the matrix becomes smaller to make it difficult to expect secondary hardening. If the amount of V-is more than 1.5 by weight, there are brought about such drawbacks that the vanadium carbide distributes to the state of network to make the weld metal brittle, and that in case the amount of C in the weld metal is 0.2 to 0.3 by weight, substantially all of the C is converted into a carbide to bring the matrix to a ferrite structure, with the result that fusionadhesion of the roll comes to be brought about. In order to facilitate the occurrence of secondary hardening of the matrix and to finely distribute the vanadium carbide, the amount of V should be within the range from 0.8 to 1.3 by weight.

M0 is also effective for prevention of hot cracking, like V, and a part of Mo forms an M C type carbide after solidification. In this case, the amount of C required is about one-twentieth the amount of Mo, so that the amount of Mo can be made larger. However, if the amount of Mo carbide becomes larger, an eutectic thereof segregates in a network state to the grain boundaries to bring about detrimental effects. According to the experiments carried out by the present inventors, the amount of M0 is preferably 1 to 4 by weight, more preferably 3 to 4% by weight.

When the amounts of Mo and V have been so decided as mentioned above, the amount of C required may also be decided accordingly. In case Mo and V are incorporated in larger amounts than those defined above, the tendency of development of a ferrite structure undesirably becomes high. In order to form carbides together with Mo and V and to dissolve a part of C into the matrix, the amount of C should be made 0.2 to 0.8 by weight. If the amount of C is less than 0.2 by weight, the resulting weldmetal comes to have a quenching hardening of less than 450 in terms of Vickers hardness (I-Iv) and is greatly deteriorated in surface roughening resistance. In' case the amount of C is more than 0.2 by weight, the C, which has dissolved in the matrix at the time of quenching, is hardened to make it possible to attain excellent surface roughening resistance. The upper limit of the amount of C is greatly dominated by the amounts of P and S contained as impurities and the amount of Si contained as a deoxidizer. However, in the case of a weld metal material, which had been prepared in consideration of the modified H.C.S. value and which had been incorporated with Ti or Al, the upper limit in amount of C was 0.8 by weight. In order to make the weld metal entirely free from any cracks, the amount of C is desirably made less than 0.6 by weight.

Ni tends to develop dendrite and has great influence on the modified H.C.S. value, so that the amount thereof is preferably as small as possible, and should not exceed 0.5 by weight in any event.

P and S segregate to the grain boundaries to greatly promote the formation of weld hot cracks, and hence should be minimized as far as possible. Permissible amounts of P and S contained in the weld metal are individually 0.015 by weight. In case the amount of each of P and S is more than 0.015 by weight, and in case the amount of C is large, the weld metal necessarily brings about hot cracking unless Mn, Cr, M0 and V are incorporated therein in large amounts.

In order to impart excellent rolling characteristics to a weld metal formed on the surface of a body, the weld metal should have a matrix structure, concretely a martensite matrix structure, to which the highest hardness can be imparted. Such structure can be attained by cooling the metal kept at a temperature capable of forming an austenite. More preferably, however, the said heat treatment should be effected after tempering the weld metal at about 720C. before it is cooled 'to less than 200C, whereby the weld metal can be prevented from formation of cold cracks.

Among weld metals formed on the surfaces of bodies, a weld metal, which displays particularly excellent rolling characteristics and which does not cause weld hot cracking, is consisted of 0.2 to 0.6 wt of C6 to 9 wt% of Cr, 3 to 4 wt% of M0, 0.8 to 1.3 wt of V,,0.l to 1.5 wt% of Si, 1 to 2 wt% of Mn, at least one of 0.01 to 0.3 wt% ofTi and 0.2 to 0.5 wt% of Al, up to0.5 wt% of Ni, up to 0.015 wt% of P, up to 0.015 wt% of S, and the balance of substantially Fe, said components other than Ti and Al having a hot crack sensitivity value of less than 2.6 when calculated according to the formula,

C(P S Si/25 Ni/30)/3Mn Cr Mo V X 10 and more restrictively', the weld metal contains no substantial Ni, and the hot crack sensitivity value thereof is less than 2.6 when calculated according to the formula,

C(P S Si/25)/3Mn Cr Mo V X 10 BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings,

FIG. 1 is a graph showing the relation between the H.C.S. value of a weld metal when calculated according to the formula proposed by Wilkinson et al. and the state of crack formation of the weld metal;

FIG. 2 is a graph showing the relation between the H.C.S value of a weld metal when calculated according to the modified formula proposed by the present inventors and the state of crack formation of the weld metal;

FIG. 3 is a slant view of the construction of a sample prepared in order to carry out hot cracking test;

FIG. 4 is a graph showing the relation between the amounts of alloying elements and the ratio of crack formation;

FIG. 5 is a slant view showing the shape of a hot finishing work roll which is an embodiment of the present invention; and FIG. 6 is a graph showing the heat treatment temper atures of weld metals formed on the surfaces of bodies.

EXAMPLE In order to investigate the influence of Al and Ti on weld hot cracking, a weld metal was evaluated in hot cracking sensitivity according to bar type cracking test. The test was carried out in the below-mentioned manner:

Two bar materials, which were individually 32 mm. in diameter and 300mm. in length, were so arranged as shown in FIG. 3, and were combined with each other by welding the two at portions distanced by 50 mm. from both ends. Subsequently, a flux and powders of alloying elements were sprinkled over the uncombinedportions and melted by application of high frequency, and the bar materials were welded with each other according to submerged arc welding using a wire which had been processed in diameter to 4 mm..After the welding, the weld metal was subjected to post-heat treatment at 700C. without lowering the temperature to below the pre-heating temperature so as to prevent the weld metal from cold cracking.

The Creek Sensitivity of the Weld metal W decided in the weld metal became about 35 of the content in Such Ii mann that the e ea g rfa of the thereof in the wire, and the content of Cr in the weld weld metal was Shaved O by 2 m. in iCk t metal'became about 45 of the content thereof in the measure the length of crack formed, and thesensitivity wi and thus somewhat higher than the content of Mo was evaluated in terms of a cracking ratio calculated or V, since more or less amount of Cr had been suppleaccording to the formula, mented from the flux.

Table 3 C Si Mn P S Cr Mo V Fe 0.28 0.59 0.9l 0-.0l 8 0.023 2.49 0.45 033 Balance Cracking ratio.(%) =Total length of crack/Total length FIG. 4 shows the relation between the amounts of alof bead X 100 loying elements and the crack ratio. As seen in FIG. .4,

d h ff d AI R Cr, Mo, Ti and A1 are effective for the reduction of Compare t e e acts 0 1 an on mac crack ratio, and the order thereof in effectiveness is Tisensittvity with those of other elements, the same test Al MO Cr as above was ff ng 0 n Cr as ll y g When viewed only from the standpoint of crack preelements. vention, Ti displays a marked effect even when used in In FIG. 3, 2 represents restriction beads welded to s h a slight amount as about 0.01 wt%; Al displays a both ends of the material 1 to be welded; 3 represents considerable effect when used in an amount of about the sprinkled flux and alloying element powders; and 4 0.04 wt%, and brings about only crater cracking when represents the welded beads. used in an amount of more than 0.05 wt%; Mo tends to The materials used in the test and the chemical comincrease the crack ratio when used in an amount. of positions thereof were as set forth in Table l. more than 4 to 5 by weight; and Cr tends to linearly Table 1 Chemical composition (wt%) Kind of material c Si I Mn v P s or Mo Ti Al Fe Materials to be welded 0.3] 0.35 0.81 0.022 0.029 0.06 Bal. Wire 0.39 0.99 0.42 0.92 0.017 0.005 5.00 1.28 Bal.

lron powder (200 mesh) 0.04 0.36 0.73 0.008 0.008 Bar Cr powder (100-150 mesh) v 0.006 0.005 Bal. Alloying Fe-Mo wder 3 I I elements 1001 0 mesh) 0.051 0.320 70.02 Bal.

Fe-Ti powder (200 mesh) 0.08 0.07 0.04 0.008 0.020 42.13 Bal. Al powder 1 (200 mesh) v Bal. L65

Welding conditions were as set forth in Table 2. reduces the crack ratio with increasing amount thereof. However, it has been confirmed that when the amounts Table 2 of Mo and Cr become more than 4 wt% and 10 wt%, respectively, an eutectic of carbides is formed to make Welding melhed Submerged are Welding the formation of heats cracks easy, so that it is desired Hux Bonded flux for hard facm Q produced that the sa d elements are added n amounts as large as by Kawasaki Iron-Making 0. possible withm the ranges. of critical values thereof. Phi-healing 200 Table 4 is an extraction of the results of welding using Weldm current 600 A 1 d f 1:- Am age 30 v severa ten in s 0 we mg wires prepared on the basis Welding rate 35 cm/min. of the results of experiments mentioned above, and 52:33: X 1 hour shows the crack sensitivities and Vickers hardnesses of Furnace coolingweld metals prepared by forming according to submerged welding six layers of each weld metal material v The alloying eiemems were d d fi it i volume on a mild steel plate of 50 mm. m thickness, 200 mm.

by addition of iron powder in order to avoid nonin width and 300 min length, y use of a wire which uniformity in thickness of the resulting weld metals was melted y Rp e of high frequency and, P

which would be encountered when the alloying ele- 955559 to 4 ,dlameterments were Small in amount The welding was effected under the conditions of a The chemical composition of a weld metal formed by current of 600 A, an arc voltage of 28 V, and a welding sprinkling only iron powder was as set forth in Table 3. rate of 40 cm/min. As the flux, there was used a mix- As seen in Table 3, the content of each of Mo and V ture of a bonded flux for hard-facing which had been produced by Kawasaki Iron-Making Co. and a bonded flux for high tensile strength steel which had been produced by Kobe Steel Works.

The presence or absence of cracks was examined by investigating the cross-section and surface of each weld metal, and the hardness of each welding was measured after quenching the weld metal from 1,000C. at a cooling rate of about 300C/hour.

Table 4 Chemical composition of weld metal Properties of weld metal to make crystal grains finer.

Ti is stronger in the above-mentioned actions, so that in case either one of Ti and Al is to be added, it is ad- C Si Mn Cr Mo V Ti Al F S Fe Modified Crack Hardness H.C.S. fonnation (Hv) 0.40 0.81 1.10 7.56 1.22 1.22 0.009 0.011 Ba]. 1.8 Internal 513 fine cks. 0.39 0.77 1.10 7.12 1.32 1.26 0.11 Bal. 1.8 4 No cks. 505 0.64 0.63 0.99 9.36 2.43 1.24 0.04 Bal. 2.4 do. 640 0.66 0.62 1.24 8.35 1.37 1.22 0.18 0.005 0.010 Bal. 2.7 do. 641 0.76 0.65 1.13 10.06 2.29 1.14 0.14 0.010 0.009 Bal. 2.7 alightk 718 1'16 C S. 0.83 0.63 1.08 9.48 2.31 1.03 0.25 Ba]. 3.1 Finecks. 755 0.85 0.70 1.22 10.05 2.14 1.15 0.18 133.1. 3.0 Large cks. 754

at surface 0.33 0.68 0.94 6.30 1.33 1.20 0.04 Bal. 1.7 No cks. 466 0.51 0.85 1.31 7.54 1.28 1.05 0.01 0.008 0.013 Bal. 2.2 Int. cks. 544 0.50 0.59 1.06 7.88 1.34 1.00 0.52 Bal. 2.2 No cks. 530 0.65 0.71 1.10 8.84 2.25 1.21 0.43 E31. 2.5 No cks. 612 0.78 0.74 1.29 9.95 2.31 1.1 l 0.50 B211. 26 Slight 628 fine cks. 628 0.78 0.66 1.28 10.03 2.20 1.18 0.96 0.005 0.019 Bal. 2.7 do. 501 0.72 0.69 1.28 9.33 2.09 1.26 l. 10 Bal. 2.6 No cks. 438 0.61 0.68 1.14 9.08 2.05 1.27 Ti+Al 0.32 0.005 0.014 Bal. 2.3 No cks. 609 0.55 0.72 1.19 7.09 1.31 1.24 0.28 Bal. 2.5 do. 583

As the result, it was confirmed that when Ti had been As is clear from FIG. 4 and Table 4, Ti is markedly incorporated, the weld metal containing 0.66-wt% of C effective even when used in a minute amount, but even and having a modified I-I .C.-S. value of 2.7 was comif Ti is incorporated in an excessively large amount, it pletely prevented from cracking, and the weld metal merely serves to form a carbide. For prevention of hot containing 0.76 wt% of C and having a modified 1-I.C.S. cracking, therefore, the amount of Ti to be incorpovalue f 2.7 f rm d slight cracks. but t gr f rated should be limited to less than 0.5 wt%. Particucrackmg was entirely negligible dependmg on the aplarly excellent effect of Ti for the prevention of hot plication purpose of the resulting work roll. In the latter 40 ki i di la d h i i incorporated i an weld metal, the m n of Cr is 1006 wt% and thus is amount of 0.01 to 0.3 wt%. If the alloying amount of Slightly excess of 10 which is the Critical h Al is less than 0.05 wt%, no substantial effect can be at- Holt/ever, Such a Sllght exeese amolfnt of CT 15 tained. According to Table 4, even the weld metal involved in the scope of the present 1nvent1on because no corporated with 004 f A] i not cause hot Change is Seen in the effectiveness of Cr. cracking. This is considered ascribable to the point that h the amount of C exceeded and the the weld metal was reduced in content of C to lower the modlfied H- l e became- 3 or more, the weld modified H.C.S. value to 1.7, whereby such favorable l f could n sufficiently be Prevented m f' result could be brought about. (This is judged from the mg in a the Contents of Cr, M0 and V are wlthm the fact that the weld metal containing 0.01 wt% of Al and aforesald ranges having a modified I-I.C.S. value of 2.2 caused internal The incorporation of Al resulted 1n no effectiveness cracking.) When the content of A1 exceeds 0.05 wt%, unless the amount thereof was made somewhat larger the cracking ratio quickly lowers, as is clear from FIG. than in the case of T1. Further, when the alloying 4, so that it is judged that the minimum amount of A1 amount of Al exceeded 1 wt%, the weld metal became required is 0.05 wt%. However, if Al is incorporated in less than 450 I-Iv in hardness and was difficult for applian excessively large amount, it dissolves in the matrix cation to hot rolling work roll. When both Ti and A1 to P m the formation of ferrite, and lowers the were incorporated, the same effect of cracking p v hardness of the weld metal to bring about a cause for tion as above was observed as well. fusion-adhesion of the resulting work roll. Thus, the Thus, so far as the amou t f C i l th 8 4% upper limit of A1 amount is 1 wt%. In order to attain a it has become possible to elevate the upper limit of the g hafdhess Withouf causing hot Cracking, A1 is P modified H C S v l e t 27 b incorporation f erably incorporated in an amount of 0.2 to 0.5 wt%. proper amounts f i and/Or AL The present inventors further attempted the incor- Both Ti and A1 have deoxidizing and denitrifying acporanfm of thls case, however, the effect of tions to reduce the amounts of oxygen and nitrogen crackmg Prevemlo" was not Sufficlemcontained in the resulting weld metal and have actions As an embodiment of the present invention, a work roll for hot finishing was produced. This roll had such a shape as shown in FIG. 5, and was composed of an body 10, a journal 11 which had been bonded to the body, and a weld metal 12 formed on the surface of the body. The total length of the roll was 2,300 mm., the diameter of the body was 300 mm., the weld metal of 40 mm. in thickness had been formed over the whole of Fe. Thereafter, the black oxide film-bearing test piece was rotated at 500 r.p.m. and the other test piece was rotated in the same direction at a rate of 4 slower than 500 r.p.m. After the lapse of 30 minutes, the rotasurface of the body, and the surface of the weld metal tion was ceased, and the degree of peeling of the black had been mechanically polished to high smoothness. The body and journal were individually composed of 0.24 wt% of-C, 0.32 wt% of Si, 0.78 wt% of Mn, 0.024 wt% of P, 0.021 wt% of S and the balance of Fe. The weld metal was composed of 0.41 wt% of C, 0.85 wt% of Si, 1.31 wt% of Mn, 0.011 wt% of P, 0.015 wt% of S, 5.01 wt% of Cr, 1.43 wt% of Mo, 0.98 wt% of V,

0.06 wt% of Ti and the balance of Fe, and had a modified H.C.S. value of 2.2.

The formation of the weld metal on the surface of the 'body was carried out in such a manner that according to submerged arc welding a wire of 4 mm. in diameter, which was composed of 0.59 wt% of C, 0.18 wt% of Si, 0.30 wt% of Mn, 0.008 wt% of P, 0.021 wt% of S, 5.33 wt% of Cr, 1.48 wt% of Mo, 1.10 wt% of V, 0.21 wt% of Ti and the-balance of Fe, was spirally welded around the body, while rotating the body, under the conditions of a welding current of 600A, an arc voltage of 30 V and-a welding rate of 35 cm/min. As the flux was used a 4:1 mixture of a flux composed of 16 wt% of SiO 16 wt% of A1 0 21 wt% of CaO, 31' wt% of MnO, 7 wt% of CaF-,, 4 wt% of Fe O and 3 wt% of M 0 and a flux composed of 20 wt% of CaCO 40 wt% of CaF 15 wt% of Mn and 25 wt% of Fe-Ti. At the time of welding, the body surface was preheated to 200 to 250C. and, during the welding, the substratum temperature was maintained at 200 to 300C. After the welding, the weld metal was immediately heated to 720C, maintained at said temperature for 2 hours, and then cooled in a furnace.

The thus produced work rollwas mechanically processed and then subjected to dye penetrant inspection test to examine the presence or absence of weld defects, but the presence of hot cracks and pin-holes was not observed at all.

The heat treatment characteristics of the abovementioned weld metal were examined by use of a separately prepared test piece to find that even when cooled at a rate of 300C/hr from 1,000C., the weld-metal could be sufficiently quenched, and that the weld metal was satisfactory also in hardness after tempering, showing a Hv value of about 500, as seen in H0. 7.

The work roll was heated at 1,000C. for 1 hour,

cooled to room temperature at a rate of 300C/hr,

again heated to 500C maintained at said temperature for 2 hours, and then air-cooled. The weld metal of the work roll thus obtained had such a structure that fine carbide particles had been distributed in the tempered martensite matrix.

In order to investigate the extent in surface roughening resistance and wear resistance of the thus produced work roll, test pieces identical in composition with the weld metal of the roll were prepared and subjected to individual tests after definite heat treatments. The suroxide film was measured. As the result, the thickness of the black oxide film became about one-half the thickness before the test, and the peeling degree was about 50 The above-mentioned test was applied also to a cast steel composed of 1.60 wt% of C, 0.50 wt% of Si, 0.30 wt% of Mn, 0.50 wt% of Ni, 0.95 wt% of Cr, 0.29 wt% of Mo and the balance of Fe which was well known as a' material excellent insurface roughening resistance. In this case, the thickness of the black oxide film became about three-tenths the thickness before the test, and the peeling degree was about The wear resistance test was effected in such a manner that a ring-shaped test piece was contacted under a contact pressure of 40 kg. withanother test piece composed of 0.29 wt% of C, 0.20 wt% of Si, 0.70 wt% of Mn, 0.029 wt% of P, 0.03 wt% of S and the balance of Fe. Thereaften the former test piece was rotated at 500 r.p.m. at room temperature, and the latter test piece was rotated in the same direction at a rate of 10 slower than 500 r.p.m. After the former test piece had rotated 6 X 10 times, the rotation was discontinued and the weight loss of the test piece due to wearing was measured. As the result, the weight loss due to wearing of the test piece was 1 g.

The above-mentioned test was applied also to a cast steel composed of 1.02 wt% of C, 0.58 wt% of Si, 0.72 wt%of Mn, 0.32 wt% of Ni, 0.90 wt% of Cr, 0.27 wt% of Mo and the balance of Fe, and to a forged steelcornposed of 0.85 wt% of C, 0.35 wt% of Si, 0.45 wt% of Mn, 2.2 wt% of Cr, 0.25 wt% of Mo and the balance of Fe. In these cases, the weight loss of the former steel was 3.75 g., and that of the latter steel was 1.8 g.

What is claimed is:

1. A work roll for use in hot rolling of a steel compris-. ing a body portion composed of a steel having a ferrite structure and having a yield strength of at least 40 Kg/mm and a working layer of weld metal welded by submerged arc welding on the surface of said body portion, said working layer having a smooth surface and having a thickness of 7 to 50 mm, said working layer consisting essentially of 0.2 to 0.8 wt% of C, 4 to 10 wt% of Cr, 1 to 4 wt% of M0, 0.5 to 5 wt% of V, 0.1 to 1.5 wt% of Si, 0.1 to 3 wt% of Mn, up to 0.5 wt% of Ni, up to 0.015 wt% of P, up to 0.015 wt% of S, at least one of 0.01 to 0.5 wt% of Ti and 0.05 to 1 wt% of Al, and the balance of substantially Fe, said components other than Ti and Al showing a hot crack sensitivity value of 2.7 or less when calculated according to the formula,

and said working layer having a structure comprising carbide particles distributed in a martensite matrix produced by maintaining said working layer at a temperature at which it is capable of forming an austenite'structure and thereafter cooling said working layer.

2. A work roll for use in hot rolling of a steel comprising a body portion composed of a steel having a ferrite structure and having a yield strength of at least 40Kg/mm' and a working layer of weld metal welded by submerged arc welding on the surface of said body -ofCr, 3 to4wt% ofMo,0.8 to 1.3 wt% ofV,0.l to 1.5

wt% of Si, 1 to 2 wt% of Mn, at least one of 0.01 to 0.3 wt% of Ti and 0.2 to 0.5 wt% of Al, up to 0.5 wt% of Ni, up to 0.015 wt% of P, up to 0.015 wt% ofS and the balance of substantially Fe, said components other than Ti and Al showing a hot crack sensitivity value of 2.7 or less when calculated according to the formula,

said working layer having a structure comprising carbide particles distributed in a martensite matrix produced by maintaining said working layer at a temperature at which it is capable of forming an austenite structure and thereafter cooling said working layer.

3. A work roll for use in hot rolling of a steel comprising a body portion composed of a steel having a ferrite structure and having a yield strength of at least 40 Kg/mm and a Working layer of weld metal welded by submerged arc welding on the surface of said body portion, said working layer having a smooth surface and having a thickness of 7 to 50 mm, said weld metal consisting essentially of 0.2 to 0.6 wt% of C, 6 to 9 wt% of Cr, 3 to 4 wt% of M0, 0.8 to 1.3 wt% of V, 0.1 to 1.5 wt% of Si. 1 to 2 wt% of Mn, at least one of 0.01 to 0.3 wt% of Ti and 0.2 to 0.5 wt% of Al, up to 0.015 wt% of P, up to 0.015 wt% of S and the balance of substantially Fe, said components other than Ti and Al showing a hot crack sensitivity value of 2.7 or less when cal culated according to the formula and said working layer having a structure comprising carbide particles distributed in a martensite matrix produced by maintaining said working layer at a tempera- .ture at which it is capable of forming an austenite structure and thereafter cooling said working layer.

4. A work roll for use in hot rolling of a steel comprising a body portion composed of a steel having a ferrite structure and having a yield strength at least 40 Kg/mm and a working layer of weld metal welded by submerged arc welding on the surface of said body portion, said working layer having a high smoothed surface and having a thickness of 7 to 50 mm, said weld metal consisting essentially of 0.2 to 0.6 wt% of C, 6 to 9 wt% of Cr, 3 to 4 wt% of M0, 0.8 to 1.3 wt% of V, 0.1 to 1.5 wt% of Si, 1 to 2 wt% of Mn, 0.01 to 0.3 wt% of Ti, up to 0.015 wt% of P, up to 0.015 wt% of S and the balance of substantially Fe, said components other than Ti showing a hot crack sensitivity value of 2.6 or less when calculated according to the formula,

C(P+S+Si/25)/3Mn+Cr+Mo+V X 10 and said working layer having a structure comprising carbide particles distributed in a martensite matrix pro-' duced by maintinaing said working layer at a temperature at which it is capable of forming an austenite structure and thereafter cooling said working layer.

5. A work roll for hot rolling a cast structure of a metal or alloy in which the cast structure is heated to above its recrystallization temperature to increase its plasticity and the cast structure is passed between said work roll and a back-up. roll in order to shape said cast structure into plates, rods or other structural shapes, said work roll comprising:

a body portion composed of a mild steel or a steel having about 0.24 wt% C, the steel of said body portion having a ferritic structure and having a yield strength of at least 40 Kg/mm and a working layer of a weld metal welded by submerged arc welding on the surface of said body portion, said working layer having a smooth surface and having a thickness of 7 to 50 mm, said working layer consisting essentially of 0.2 to 0.8 wt% ofC, 4 to 10 wt% of Cr, 1 to 4 wt% of M0, 0.8 to 1.3 wt% of V, 0.1 to 1.5 wt% of Si, 1 to 2 wt% of Mn, at least one of 0.01 to 1.3 wt% of Ti and 0.2 to 0.5 wt% of Al, up to 0.5 wt% of Ni, up to 0.015 wt% of P, up to 0.015 wt% of S and the balance of substantially Fe, said components other than Ti and Al showing a hot crack sensitivity value of 2.7 or less when calculated according to the formula,

and said working layer having a structure comprising carbide particles distributed in a martensite matrix produced by maintaining said working layer at a temperature at which it is capable of forming an austenite structure and thereafter cooling said working layer.

6. A work roll according to claim 5, wherein said body portion is composed of a mild steel.

7. A work roll according to claim 5, wherein said body portion is composed of a steel consisting essentially of about 0.24 wt% C, about 0.32 wt% Si, about 0.78 wt% Mn, about 0.024 wt% P, about 0.02 wt% S and the balance Fe.

8. A work roll according to claim 5, wherein the structure of said working layer is produced by heating said weld metal to a temperature of about 720 C and thereafter cooling.

9. The work roll of claim 8, wherein said weld metal is maintained at a temperature of about 720 C for about 2 hours before being cooled. 

1. A WORK ROLL FOR USE IN HOT ROLLING OF A STEEL COMPRISING A BODY PORTION COMPOSED OF STEEL HAVING A FERRITE STRUCTURE AND HAVING A YIELD STRENGTH OF AT LEAST ABOUT 40KG/MM2, AND A WORKING LAYER OF WELD METAL WELDED BY SUBMERGED ARC WELDING ON THE SURFACE OF SAID BODY PORTION, SAID WORKING LAYER HAVING A SMOOTH SURFACE AND HAVING A THICKNESS OF 7 TO 50 MM, SAID WORKING LAYER CONSISTING ESSENTIALLY OF 0.2 TO 0.8 WT% OF C, 4 TO 10WT% OF CR, 1 TO 4 WT% OF MO, 0.5 TO 5 WT% OF V, 0.1 TO 1.5 WT% OS SI, 0.1 TO 3 WT% OF MN, UP TO 0.5 WT% ON NI, UP TO 0.015WT% OF P, UP TO 0.015WT% OF S, AT LEAST ONE OF 0.01 TO 0.5 WT% OF TI AND 0.05 TO 1 WT% OF AL, AND THE BALANCE OF
 2. A work roll for use in hot rolling of a steel comprising a body portion composed of a steel having a ferrite structure and having a yield strength of at least 40Kg/mm2, and a working layer of weld metal welded by submerged arc welding on the surface of said body portion, said working layer having a smooth surface and having a thickness of 7 to 50 mm, said weld metal consisting essentially of 0.2 to 0.6 wt% of C, 6 to 9 wt% of Cr, 3 to 4 wt% of Mo, 0.8 to 1.3 wt% of V, 0.1 to 1.5 wt% of Si, 1 to 2 wt% of Mn, at least one of 0.01 to 0.3 wt% of Ti and 0.2 to 0.5 wt% of Al, up to 0.5 wt% of Ni, up to 0.015 wt% of P, up to 0.015 wt% of S and the balance of substantially Fe, said components other than Ti and Al showing a hot crack sensitivity value of 2.7 or less when calculated according to the formula, C(P+S+Si/25+Ni/30)/3Mn+ Cr+Mo+V X 103 said working layer having a structure comprising carbide particles distributed in a martensite matrix produced by maintaining said working layer at a temperature at which it is capable of forming an austenite structure and thereafter cooling said working layer.
 3. A work roll for use in hot rolling of a steel comprising a body portion composed of a steel having a ferrite structure and having a yield strength of at least 40 Kg/mm2, and a working layer of weld metal welded by submerged arc welding on the surface of said body portion, said working layer having a smooth surface and having a thickness of 7 to 50 mm, said weld metal consisting essentially of 0.2 to 0.6 wt% of C, 6 to 9 wt% of Cr, 3 to 4 wt% of Mo, 0.8 to 1.3 wt% of V, 0.1 to 1.5 wt% of Si, 1 to 2 wt% of Mn, at least one of 0.01 to 0.3 wt% of Ti and 0.2 to 0.5 wt% of Al, up to 0.015 wt% of P, up to 0.015 wt% of S and the balance of substantially Fe, said components other than Ti and Al showing a hot crack sensitivity value of 2.7 or less when calculated according to the formula C(P+S+Si/25)/3Mn+Cr+Mo+V X 103 and said working layer having a structure comprising carbide particles distributed in a martensite matrix produced by maintaining said working layer at a temperature at which it is capable of forming an austenite structure and thereafter cooling said working layer.
 4. A work roll for use in hot rolling of a steel comprising a body portion composed of a steel having a ferrite structure and having a yield strength at least 40 Kg/mm2, and a working layer of weld metal welded by submerged arc welding on the surface of said body portion, said working layer having a high smoothed surface and having a thickness of 7 to 50 mm, said weld metal consisting essentially of 0.2 to 0.6 wt% of C, 6 to 9 wt% of Cr, 3 to 4 wt% of Mo, 0.8 to 1.3 wt% of V, 0.1 to 1.5 wt% of Si, 1 to 2 wt% of Mn, 0.01 to 0.3 wt% of Ti, up to 0.015 wt% of P, up to 0.015 wt% of S and the balance of substantially Fe, said components other than Ti showing a hot crack sensitivity value of 2.6 or less when calculated according to the formula, C(P+S+Si/25)/3Mn+Cr+Mo+V X 103 and said working layer having a structure comprising carbide particles distributed in a martensite matrix produced by maintinaing said working layer at a temperature at which it is capable of forming an austenite structure and thereafter cooling said working layer.
 5. A work roll for hot rolling a cast structure of a metal or alloy in which the cast structure is heated to above its recrystallization temperature to increase its plasticity and the cast structure is passed between said work roll and a back-up roll in order to shape said cast structure into plates, rods or other structural shapes, said work roll comprising: a body portion composed of a mild steel or a steel having about 0.24 wt% C, the steel of said body portion having a ferritic structure and having a yield strength of at least 40 Kg/mm2; and a working layer of a weld metal welded by submerged arc welding on the surface of said body portion, said working layer havinG a smooth surface and having a thickness of 7 to 50 mm, said working layer consisting essentially of 0.2 to 0.8 wt% of C, 4 to 10 wt% of Cr, 1 to 4 wt% of Mo, 0.8 to 1.3 wt% of V, 0.1 to 1.5 wt% of Si, 1 to 2 wt% of Mn, at least one of 0.01 to 1.3 wt% of Ti and 0.2 to 0.5 wt% of Al, up to 0.5 wt% of Ni, up to 0.015 wt% of P, up to 0.015 wt% of S and the balance of substantially Fe, said components other than Ti and Al showing a hot crack sensitivity value of 2.7 or less when calculated according to the formula, C(P+S+Si/25+Ni/30)/3Mn+Cr+Mo+V X 103 and said working layer having a structure comprising carbide particles distributed in a martensite matrix produced by maintaining said working layer at a temperature at which it is capable of forming an austenite structure and thereafter cooling said working layer.
 6. A work roll according to claim 5, wherein said body portion is composed of a mild steel.
 7. A work roll according to claim 5, wherein said body portion is composed of a steel consisting essentially of about 0.24 wt% C, about 0.32 wt% Si, about 0.78 wt% Mn, about 0.024 wt% P, about 0.02 wt% S and the balance Fe.
 8. A work roll according to claim 5, wherein the structure of said working layer is produced by heating said weld metal to a temperature of about 720* C and thereafter cooling.
 9. The work roll of claim 8, wherein said weld metal is maintained at a temperature of about 720* C for about 2 hours before being cooled. 